On 01/04/2012 15:28, Hugo Nebula wrote:
Reading car forums, the overwhelming opinion is that a pressure washer
is needed to wash one's car properly. So I went and bought a cheap
Halfords one, complete with snow foam attachment. The results were
disappointing to say the least.

The dirt on cars is known as 'traffic film' and comprises of unburnt
fuel residue, oil/grease residue, small amount of rubber & general
grime. It's basically oil based & forms an extremely good bond to
vehicle paintwork. In addition, since the vehicle is moving while it
gets dirty, a static bond is also generated.

It short, it's a bugger to remove without mechanical agitation.

Pressurised water alone will not shift traffic film. A specific
detergent - known as a Traffic Film Remover or TFR is needed to remove
it. You won't find a decent product in a DIY place, proper TFR is
pretty nasty stuff - pH of around 13. You need to go to a specialised
pressure washer dealer.

I know it's not a mega powerful one, but I was amazed at how low the
pressure was out of the nozzle. I was certain my hose gave a more
powerful flow, so I did an experiment and timed how long it took to
fill a bucket, and therefore what the flow rate is from my tap. It
amounts to 1,000 litres per hour, nearly three times the flow rate of
the PW. Some of the best pressure washers only appear to give half of
that.

Pressure washers were developed in Denmark & Germany when water & it's
disposal has always been very expensive. The concept was that you could
do the same amount of cleaning with much less water. In the UK, where
water has been cheaper, they were sold as labour saving, not water saving.

Am I missing something here? Is pressure a function of the flow rate,
and if so, what's the point of a pressure washer if it gives less than
mains pressure?

You are confusing flow & pressure. A household tap may give 5 bar at 15
litres a minute. Connecting a pressure washer to that tap gives say 100
bar at 10 litres/min - which is a tenfold increase in cleaning power.

The balance of pressure/flow is important. 120 bar @ 6.5 l/min is less
powerful than 100 bar @ 8 l/min.

Have a look here;

http://wiki.diyfaq.org.uk/index.php?...ure_washer_FAQ

--
Dave - The Medway Handyman www.medwayhandyman.co.uk